Amplifiers for amplifying voltage or current signals are used in a diverse range of applications ranging from subsonic frequencies to a GHz range for advanced wireless communications systems. Transducer amplifiers are one important type of amplifier where it is required to detect, amplify and otherwise process electrical signals representative of a physical quantity or variable such as sound pressure, acceleration, temperature, pressure, etc. Transducer amplifiers are used in miniature microphones and speakers for mobile terminals and other portable communication devices such as mobile phones, headsets and hearings instruments. Portable devices are typically operated on rechargeable batteries and comprise a large number of power-consuming electronic components. This makes it of the outmost importance to lower power consumption of the transducer amplifier to the lowest possible level compatible with stated or given performance requirements. In many situations there may be a fixed amount of available power, or a power budget, for the transducer amplifier.
The above-mentioned performance requirements typically impose certain constraints on the amount of power or current that a specific transducer amplifier requires. For example, a requirement for a certain input noise voltage leads to a minimum bias current requirement for an input stage of the transducer amplifier once circuit topology and semiconductor technology have been chosen. Likewise, a requirement for a certain load handling capability of an output stage of the transducer amplifier leads to a minimum bias current requirement for an output stage of the transducer amplifier once circuit topology, for example class A, AB or B, and semiconductor technology have been chosen.
However, in accordance with the present invention a level of DC bias current supplied to the input stage relative to the level of DC bias current supplied to the output stage of the amplifier is dynamically adjusted in response to an input signal level at the input stage. In an advantageous embodiment, the total power consumption of the transducer amplifier is maintained at an essentially constant level to comply with the fixed power budget. The dynamic adjustment of DC bias provides performance improvement in the transducer amplifier because the DC bias current in the input stage can be reduced under large transducer signal conditions without audible noise degradation of the amplified transducer signal due to perceptual masking. At the same time, increasing the DC bias current, and therefore power consumption, of the output stage, under large signal conditions improves the load handling capability which is required to provide appropriate output voltage and current for the load.